Fibreboard is a type of composite wood panel made from lignocellulosic fibres, combined with a resin or other suitable binder, and then bonded together under heat and pressure. Fibreboards are classified by density, for example a fibreboard with density between 500 kgm−3 and 800 kgm−3 is classified as medium density fibreboard (MDF) and a fibreboard with specific gravity greater than 800 kgm−3 is classified as high density fibreboard (HDF).
Fibreboards are manufactured primarily for use as panels, insulation and cover materials in buildings and construction where flat sheets of moderate strength are required. They have frequently taken the place of solid wood, plywood, and particleboard for many furniture applications. They are also used to a considerable extent as components in doors, cabinets and cupboards. The potential use of fibreboard in other interior and exterior markets such as mouldings, exterior trim and pallet decking has been explored by the industry and the market is fast expanding.
Fibreboards are manufactured via a multi-step process. Typically, wood chips or other suitable materials are softened and lignin is broken down. The treated chips are then separated into their component fibres, and the hot, wet fibres are rapidly transported via steam through a blowline in which the binders are added to the fibres via blowline injection. The blowline deposits the binder-treated fibres into a dryer, and eventually into forming and pressing devices which produce the final panels. Ideally, polymerization of the binder into its final thermoset form takes place in the hot press, not prior to pressing the fibreboards into their final form. To avoid the problem of binder sticking on the press platens, a low dosage of wax emulsion has to be added together into the binders.
Traditional binder resins used in the manufacture of fibreboards include thermosetting binders, especially formaldehyde-based binders, such as urea formaldehyde and melamine formaldehyde. The binder content could range from 10% to 20% of total finished fibreboards weight. Isocyanate-based binders, particularly methyl diphenyl diisocyanate (MDI) binders, have some significant advantages over formaldehyde-based binders, including superior physical and moisture-resistance properties as well as the ability of eliminating formaldehyde emission hazards. Although isocyanate-based binder technology for fibreboard manufacture has been available for many years, this binder, however, has not gained widespread commercial acceptance, primarily because of its cost, coupled with difficulties countered in process design.
The use of isocyanate-based binders via blowline injection also poses different drawbacks. This is due to the fact that isocyanates react very quickly with water to form polyureas at elevated temperatures, such as the temperatures experienced in the blowline. It may cause a great deal of isocyanate being converted to polyureas prior to pressing, that can lead to the formation of solids, which foul the blowline or the dryer. Besides, pre-polymerization renders a significant portion of the binder non-effective, greatly reducing bonding efficiency. Isocyanate is also volatile and may be lost in the drying process at high temperatures.
In order to protect isocyanate-based binders from the harsh conditions of the typical fibreboards manufacturing process, emulsified isocyanates have been developed. These modified isocyanates can be mixed with water using an in-line static mixer, and subsequently injected into the blowline. Emulsification prevents the build up of isocyanate-water reaction products on the walls of the blowline and helps to prevent premature reaction of the binder. Emulsification also helps to increase the volume of liquid being dispensed, which helps to achieve a more even distribution of binder throughout the fibres. Unfortunately, converting an isocyanate to the emulsified form entails additional manufacturing costs, which make these products less cost effective. In addition, the presence of emulsifying agents could also affect the properties of the boards, as it is well known that many surfactants used as emulsifying agents could absorb moisture and reduce the bonding strength of the binder resins.
Another cost-disadvantage of isocyanate binders is their need for a release agent, in order to avoid sticking to metal press platens. In industrial practice, water-based release agents are typically emulsified in line with the emulsifiable MDI just prior to blowline injection.
All of the technologies set forth in the preceding description are well-known. For example, the process for using isocyanate binders for the production of fibreboards via blowline injection is described in U.S. Pat. No. 4,407,771 issued to the Celotex Corporation in 1983. An example of the use of emulsified isocyanate binders is described in U.S. Pat. No. 3,996,154 issued to ICI Americas Inc. in 1976. The use of wax release agents in conjunction with isocyanate binders is described in U.S. Pat. No. 4,388,138 and U.S. Pat. No. 4,396,673, both issued to ICI Americas Inc. in 1983. In addition, the use of an in-line mixing apparatus for dispensing isocyanate binders into the blowline of a manufacturing process is described in, for example, U.S. Pat. No. 5,093,058 issued to the Medite Corporation in 1992.
Various types of fibreboard based on natural materials and methods for manufacturing thereof have been disclosed in the prior arts. Japanese Patent No. 2002283313 relates to a method for manufacturing fibreboard in which raw fibre was obtained from kenaf, oil palm or coconut. However, the binder resins used are water-soluble phenol adhesives. Another Japanese Patent No. 11264212 also relates to a fibreboard made from palm fibre. Similarly, the curing resin used includes phenolic resins. Another type of binder is the amino resin deriving from the reactions of urea or melamine with formaldehyde as disclosed in Japanese Patent No. 11198110. All these patented technologies only use natural fibres as the raw materials but not natural oil-based binder.
Despite all of these known technologies, the use of isocyanates in fibreboard manufacture had remained unpopular, because of the limitations of the aforementioned cost effectiveness and related application problems. As the isocyanates offer significant advantages in product properties over other technologies to be used as binder in the manufacture of fibreboards, it is therefore desirable for the present invention to apply an innovated type of isocyanate-based binder in the manufacture of fibreboard. The natural oil-based binder can be obtained from an improved method using more cost effective materials, such as environmentally friendly natural materials.